Initial and End date BPMN task

Good morning,

I wander if anyy of you could help me with this. 

I need to find a way to record the initial and end date of a Task in BPMN. I am thinking about using an object attributte, but the standard ones just allow the complete date (meaning mm-dd-yyyy) when what I need it is just the day and the month the task is performed or the day of the month is performed

I.e, the task is started the 1st March of each year and ends the  the 15th of that month. Or the tasks is started the 1st day of each month and ends the  the 15th of the same month. I want to record it this way to be able to extract a report sorting all the yearly tasks of the different teams.

I may use a "dummy year", but I'd rather not.

Any ideas?

Best regards

David izquierdo